1. Field of the Invention
The invention relates generally to improvements in the field of windrow elevators used in an asphalt paving machine to pick up, process, and provide a ready supply of asphalt-based material. More specifically, the present invention comprises an apparatus and a method, for fragmenting agglomerated pieces of rubberized asphalt material and re-mixing the fragmented pieces with smaller pieces of the same material, to achieve an acceptably homogeneous consistency in the material readied for immediate use by the asphalt paving machine.
2. Description of the Prior Art
A material commonly known as Hot Mix Asphalt (“HMA”) is widely used in roadway construction and resurfacing. HMA is comprised of a mixture of asphalt oil binder, sand, small rocks, and other filler material, processed at a batch plant. Ideally, the batch plant is located close to the paving site, so the HMA will stay hot and workable until it is applied on the roadway. A short transportation distance also minimizes the phenomenon known as material segregation. Because HMA is composed of different sized aggregate and fill material, agitation and gravity act on these pieces of HMA differently. The larger, heavier pieces and the smaller, lighter pieces tend to separate and collect in like groups during transport. When the dump trailer deposits the HMA material on the roadway in a windrow, the smaller particles are concentrated in the central, elevated region of the windrow and the larger particles are concentrated in the lateral, lower regions of the windrow.
U.S. Pat. No. 6,481,922, issued to Boyd, provides a solution to the above-noted material segregation problem. The '922 Patent discloses an apparatus and a method for re-mixing the large particles with the small particles, so that a more uniform mixture of those particles is achieved before the HMA is applied onto the roadway. Through the use of a pair of lateral augers which continuously deliver the larger particles into a centrally positioned stream of the smaller particles, the HMA is re-mixed into a homogeneous mixture before being delivered into a collection hopper for subsequent application on the roadway.
However, as a roadway material, HMA is not without its faults. The asphalt oil binder used to coat and hold the aggregate particles together, plays a critical role in the performance and longevity of the roadway. The adhesive and agglomerating properties of the binder are affected by temperature, the amount and rate of road loading, and aging. Over a period of time, the surface of well-used roads, particularly in harsh environments, begins to crack and delaminate from lower support layers. To mitigate these effects, various additives have been proposed and tested with existing asphalt binders.
One of the most promising and successful additives used so far is rubber, recycled from used motor vehicle tires containing a high content of natural rubber. The tires are ground up into small particles known in the industry as “crumb rubber”. Pieces of the steel belts used in the manufacture of tires are removed from the ground up tires, before the crumb rubber is ready for further processing and incorporation into an asphalt based road material. The resultant product is variously known in the paving industry as Rubberized Asphalt, Rubberized Asphalt Concrete (“RAC”), and Asphalt-Rubber.
Two basic methods have been used to make the rubberized asphalt. The first method, known as the “wet process”, calls for the crumb rubber to be mixed with the binder (approximately 80% asphalt cement and 20% rubber) in a field blending unit. This first step occurs prior to the addition of the mixture to the other materials at a separate hot mix plant. In a second method, rubberized asphalt can be produced directly, using a terminal blended process where the crumb rubber is added at the refinery or at the asphalt cement terminal. The advantage of the latter method is that no specialized and costly rubberized blending plant is required, and the asphalt binder can be shipped to the hot mix plant just as a standard binder would be.
Irrespective of how it is manufactured, rubberized asphalt has been proven a superior roadway material over unmodified HMA in several significant areas. Rubberized asphalt is highly-skid-resistant, quieter than HMA or concrete, and resistant to rutting and cracking. In the process of making rubberized asphalt, used tires are consumed and utilized for a new purpose. A two-inch thick roadway resurfacing project can consume approximately 2000 waste tires per lane per mile. Thus, land-fill can be reduced and environmental concerns associated with the storage of flammable stores of waste tires are alleviated. Research has established that 4″ thick conventional HMA roadway can be replaced with 2″ thick rubberized asphalt, and achieve the same fatigue life. Rubberized asphalt provides excellent long-lasting, color contrast, for road striping and marking. Lastly, rubberized asphalt can generally be applied using conventional road-paving equipment and methods.
The last mentioned feature of rubberized asphalt has several exceptions, however. Rubberized asphalt is made using smaller and more uniform aggregate, typically on the order of ¼″ to ⅜″, or so, in diameter. This results in a material which is much less susceptible to the segregation problem caused by material transport, characteristic of HMA. But rubberized asphalt cools at a different rate than HMA, and it has a tendency to agglomerate in ways that HMA does not. Between the batch plant where the rubberized asphalt is manufactured and the roadway job site, cooling of the material occurs, especially in areas contingent and adjacent the sidewall and floor of the material hopper.
When rubberized asphalt has cooled a sufficient amount before it is even deposited into a windrow on the roadway, it may agglomerate into relatively large balls or sheets of material of irregular size and shape known as “clingers”. For example, a sheet of such agglomerated material may be 2″ to 3″ thick, 4″ to 5″ wide, and 12″ to 18″ long. These large pieces of agglomerated material are randomly dispersed through the windrow.
The present practice is to remove such agglomerated material manually from the windrow, before material pickup and application of the rubberized asphalt to the roadway occurs. This method is labor intensive, and also relies upon the workers finding and removing all of the offending clingers. If not removed, such large chunks of agglomerated material may jam in the paving machine or be deposited into the roadway and remain an unintegrated surface component.